1. Field of Invention
This invention relates to cationic conjugated polyelectrolyte electron injection layers.
2. Related Art
Charge injection and transport play an important role in organic light emitting diodes (OLEDs) (1,2). In OLEDs, holes are injected from the anode into the highest occupied molecular orbital (HOMO) of the organic semiconductor. Similarly, electrons flow from the cathode to the lowest unoccupied molecular orbital (LUMO). A balance of injection and transport of both charge carriers is needed to increase the probability of hole/electron recombination, and hence, improve the light output at a given current density. In the absence of interfacial effects one needs to match the energies of the HOMO and the LUMO with the work function of the anode and cathode, respectively, so to minimize the injection barriers. Stable metals have high work function that give rise to large electron injection barriers. One therefore needs to rely on multilayer devices or the use of less stable low work function cathodes such as barium and calcium (3,4).
Recently, conjugated polyelectrolytes (CPEs) have been shown as effective electron injecting layers (EIL) in polymer LEDs (PLEDs) leading to enhance performances with environmentally stable cathodes such as Au, Ag, Cu and Al (5-12). CPEs are composed of a π-conjugated backbone with pendant groups bearing ionic functionalities. The ionic functionalities allows the fabrication of multilayer devices by solution process without significant perturbation of the underneath layer by taking advantage of the solubility of CPEs in polar solvents (13). The operating mechanism for the reduction of electron injection barrier remains under debate with one model predicting the formation of permanent interfacial dipoles between the cathode and the CPE (5). Another model incorporates the notion of ion migration by the applied electric field, leading to redistribution of the internal field within the CPE EIL, in addition to the contribution of interfacial dipoles (7).
A number of CPE structures have been reported with a variety of conjugated backbones, appended ionic functionalities and counter ions. These materials lead to a variety of device performances that are not fully understood)9-11). Recently, a systematic investigation of various conjugated backbones on device performance was reported with direct correlation of the electron mobility of the CPE EIL with device performance (9). Organic counter ion alteration of CPEs has also been shown to lead to orders of magnitude differences in device performance (12). Though the behavior of CPE EILs in PLEDs is attractive for inexpensive all solution processed commercial applications (13) the CPEs properties influence on device performance and optimization is not fully understood.